Eventually in the future, bitcoin will be replaced by another crypto currency but not soon. Bitcoin will be available till 2023 for mining. After that bitcoin will remain the costliest crypto currency. However, those who have already set up their expensive mining system, what will they do?
They will obviously find another crypto currency to mine and that crypto currency will become the next big thing after bitcoin. Everything in life, has a expiry date and bitcoin is no exception. But that is not going to happen anytime soon.
You will have to explain some details. Why mining will only be available until 2023? Because we should still have a good 100 years until no new blocks are mined. Also the fees will take over as the reward. Altcoins use different algorithms, with ASCIs it is not possible to switch to the next best coin.